Find a resistor marked as R37 on the PCB:
Solder a 10,000Ohm variable resistor to this R37 or draw over it with a lead pencil. The original resistance is 1150Ohms. You can now reduce it in steps of 115-125Ohms to increase the core voltage in a step of 0.1V.
The core voltage is monitored on the legs of the top row of capacitors.
The default voltage on my sample of the card was 1.3V. I increased it to 1.55V and achieved a rather high core frequency of 745MHz.